Re: Dinosaur Revolution Review

[Jura <pristichampsus@yahoo.com>]

One last time (this time just the relevant cut portions)


Scott Hartman wrote:

If one insists upon a
> hardline "mutual exclusivity" hypothesis like Jason advocates then you
> must assume that the line to feathered dinosaurs had to first evolve
> naked patches of skin before any form of dino-fuzz could even exist.
> I can't say that's impossible, but it seems much more plausible (to
> me) that there were many permutations of co-existing feathers and
> filamentous structures (perhaps some of them like those seen  in
> Psittacosaurus and Tianyulong).  With scales and filaments co-existing
> (perhaps at times just restricted to different parts of the body) then
> losing them in lineages is simple, and does not require the
> re-evolution of scales in those areas.  It could be that only after
> truly extensive feathering evolved (perhaps even after flight evolved
> and weight-savings became more of an issue) that there was a selective
> pressure that favored making scales and feathers developmentally
> exclusive (a condition that is still easily overcome even today via
> artificial selection).


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Just
 to clarify, the evolution of feathers from scales would not actually 
require a naked stage. Our current understanding of feather 
morphogenesis suggests that feathers came about by hijacking the scale 
developmental pathway. In this case, all that would need to happen would
 be one, or a few mutations (e.g. the scale beta keratin codon deletions
 that lead to the feather beta keratin) to cause what would look like a 
global change to the entire integument. The scale pathway would 
continue, but scales would no longer form, and in their place we would 
have filaments. Natural selection could fine tune it from there.

Going
 back from feathers to scales would be more difficult and might have 
"required" a naked skin stage, but judging from what we currently know 
about scale formation in birds, it too could have occurred as a couple 
of mutations that ultimate
on the
 cascade that lead to integument development on the tarsometatarsus.

Jason